The production of high strength steel is performed at present through the following methods:
a--controlled rolling; PA0 b--normalization; PA0 c--conventional quenching and temper.
For the purpose of increasing the cost/quality ratio of the product, the method of controlled rolling, for a well defined range of thicknesses, is preferable as it permits the best balance between costs for the chemical composition, production times, and quality.
The normalization method has remarkable limits in productivity, in a thermal treatment cycle which is performed after rolling in a `cold` sheet. In fact, as is well known, in the controlled rolling method the mechanical characteristics are obtained at the same time as the physical transformation through a thermomechanical treatment.
Even for compromise between cost, chemical composition, and mechanical properties, the normalized product is less advantageous than a product obtained through the controlled rolling method.
On the contrary, the quenching and tempering treatment obtains quality levels absolutely higher than those of both the controlled rolling and the normalization methods, and further for the same quality has a cost balance for the chemical composition/mechanical properties much more favourable than the other two methods.
However, in consequence of the double thermal cycle on a `cold` sheet after the rolling and the quick cooling of the quenching step, the ratio (production cost/quality) is not competitive with the ratios deriving from the other two treatments, at least for quality levels corresponding to an ultimate tensile strength (UTS) greater than 60 kgs/mm.sup.2 on thicknesses up to 50 mm for the controlled rolling and 100 mm for the normalization. For higher levels no recourse to other methods other than the quenching and temper one is known in the industry.
Recent studies of the Metallurgy Division of the British Iron and Steel Research Association and other laboratories have pointed out the possibility of obtaining, in particular types of steels, improvements in the strength and toughness by a quenching treatment performed right after the controlled rolling (direct quenching), with respect to natural cooling. The favourable effects remain even after tempering, so the characteristics are improved even with respect to the quenching and tempering method performed through a prolonged heating in austenization furnaces before the forced cooling.
That involves the performance of the quenching within the first seconds after the last passage of the controlled rolling, within a period of time such as to allow the temperature of the material to remain within in the austenitic range.